1. Field of The Invention
This invention relates to devices used in gamma radiography. More particularly, this invention relates to a calculator and related method of solving for one of five gamma radiographic variables useful in developing radiographs.
2. Description of The Related Art
Gamma radiography is useful for performing nondestructive tests of various objects, including structural members, such as reinforced concrete beams and columns, steel girder welds, reinforced concrete walls and floors, piping, pressure vessels, castings and the like. Gamma radiography involves irradiating an object of interest with a gamma radiation source (e.g., Iridium 192 (Ir.sup.192), Cobalt 60 (Co.sup.60), etc.). The gamma particles from the gamma radiation source pass through, or are absorbed by, the object of interest and strike a photographic film target. The latent image on the film target, also known as a radiograph or radiogram, is a 2-dimensional representation of the density of the object of interest. Generally, highly exposed (dark) regions of the radiograph indicate low density, and conversely, underexposed (light) regions represent high density. Thus, undesirable voids or discontinuities in a steel girder weld might appear as a dark region where a light region should have appeared.
When making a radiograph, several variables must be taken into account, including thickness and material of the object of interest, the type of film used to develop the radiograph, the distance between the radiation source and the film target, the intensity of the radiation source (or source strength), the desired film exposure density, the length of time of the exposure and the relative strength or weakness of the chemicals being used to develop the radiograph.
One prior art approach to solving for exposure time using either Ir.sup.192 or Co.sup.60 radiation sources is the GAMMA RADIATION EXPOSURE CALCULATOR, from JEM Manufacturing Corporation. Using this prior art slide-rule device, one matches the desired exposure density to the age of the radiation source. Then the source-to-film distance is matched against a steel thickness scale. Finally, the exposure time is read from the corresponding radiation source strength. One drawback with this apparatus is that one must know the correct exposure per hour (in Roentgens) for the particular type of film being used. Put another way, if the user of this apparatus only knows what type of film is being used, and not the radiation dose, he cannot calculate exposure time. The apparatus is also incapable of adjusting for strong or weak developing chemicals.
A circular calculator for the solution of radiation penetration problems is disclosed by U.S. Pat. No. 3,700,162 to Gaggero et al. The Gaggero et al. patent teaches solving for one of five variables given the other four. The five variables of interest in the Gaggero et al. patent are: source intensity, I, usually measured in Curie, sometimes in Becquerel; exposure time, T, usually measured in minutes; thickness of the object of interest, L, source-to-target distance, K, typically measured in inches; and radiation dose at the target (film) position, R, typically measured in Roentgens, or sometime in Seiverts. As disclosed in Gaggero et al., R can be expressed as a function of the other four variables: EQU R=I.multidot.T.multidot.K.sup.-2 F(L), (1)
where F(L) is the radiation dose for unit values of the variables I, T and K. F(L) depends on the energy spectrum of the radiation source, and on the material composition of the target of interest. The Gaggero et al. patent does not disclose the solution of exposure time, T, from variables I, K, L, and film type. Furthermore, Gaggero et al. notes that "type of film" and "developing conditions" are variables that affect quick determination of exposure time required to obtain radiographs of good quality. However, Gaggero et al. suggests that such parameters as "type of film" and "developing conditions" are "not continuously varying parameters and hence they are somewhat difficult to handle" thus, teaching away from their use.
Thus, there is a need in the art for methods and apparatus for calculating gamma radiography variables which employ or consider the variable of film type, and which compensate for film developing conditions (i.e., relative chemical strength).